This instructs the compiler to stop parsing the current file, open and completely parse the include file, the return to this file on the next line. Note the included file must have an extension of '.jal' and the filename may not begin or end with a space.
Note that it is not possible to include the same file multiple times. Once a file is included, it will not be included again. Also be aware that the filename is taken literally – no transform is done on it. This should be taken into consideration if you are writing a library as some filesystems are case-sensitive, and others are not, so "MYLIBRARY" and "mylibrary" might be two different files.